Recently, from the view points of the dangerousness of an inflammation, an explosion, etc., the toxicity to human being, the occurrence of environmental pollution, etc., caused by organic solvents, the tendency of using aqueous resins in place of organic solvent type resins conventionally used has been increased. In particular, in regard to aqueous polyurethane resins, polymers having characteristic excellent properties are obtained by reacting many isocyanate compounds and polyol compounds each having different structures together with a chain-lengthening agent from soft elastomers to hard plastics, various investigations for obtaining aqueous polyurethane resins have been proceeded in a wide field of coating materials, adhesives, impregnants, etc.
As aqueous polyurethane resins, anion type resins, cation type resins, and nonion type resins are known but from the points of the stability with the passage of time, the physical properties thereof, the miscible stability with other emulsions or various kinds of pigments, etc., a self-emulsifying aqueous polyurethane having a carboxyl group introduced into the polyurethane resin skeleton has been watched with keen interest.
An ordinary method of introducing a carboxyl group into a polyurethane resin is practiced by reacting a polyol compound with an excessive amount of a polyisocyanate compound to synthesize an isocyanate group-terminated prepolymer and then reacting the prepolymer with a compound having at least one carboxyl group and two active hydrogen atoms capable of reacting with isocyanate groups in the molecule.
As a compound for introducing a carboxyl group into a polyurethane skeleton, the compounds described in D. Dieterich, Progress in Organic Coatings, 4, 281-340 (1981), etc., are known but these compounds have the problems that they all have a high melting point, the solubility thereof in polyurethane resins and organic solvents being used for polyurethane resins is poor, and the introduction of a carboxyl group into the resin skeleton is not easy.
As a compound capable of relatively easily introducing a carboxyl group into the polyurethane resin skeleton in conventionally known compounds, there is 2,2'-dimethylolpropionic acid but the compound has a high melting point and since the solubility of the compound in polyurethane resins and organic solvents is poor, it is required to use an organic solvent having a high polarity such as N-methyl-2-pyrrolidone in the case of using the foregoing compound. However, it is difficult to remove N-methyl-2-pyrrolidone, etc., which is a water-soluble high boiling solvent, from an aqueous polyurethane resin and hence there is a problem that the aqueous polyurethane resin obtained by the method is used in the state of containing the organic solvent.